1. Field of the Invention
The present invention relates to the use of solder in printed wiring cards, and more particularly to the addition of copper doping of eutectic solders utilized in soldering printed wiring cards.
2. Background Art
In the field of screen printing copper conductors on printed wiring cards, it has been found desirable to utilize a low temperature thick film cermet copper conductor. Typical of such conductor material is the number 6001 copper, manufactured by E. I. DuPont.
In the usual arrangement, the copper conductors are screen printed over an alumina (aluminum oxide) substrate and dried at approximately 120 degrees centigrade to remove the organic vehicle, followed by sintering at approximately 600 degrees centigrade using a conveyor oven equipped with a nitrogen atmosphere with the addition of approximately 5 to 15 ppm of oxygen in accordance with the manufacturer's instructions. This conductive material is typically utilized to form pads for component mounting and also to form the necessary electrical interconnects between the circuit elements.
Thick film cermet copper conductors like the aforementioned DuPont number 6001 are difficult thick film materials to solder. It has been found that the conductor has a tendency to leech and form copper tin intermetallics (i.e., dewet about the perimeter of the conductor pad), or to produce areas of non-wetting depending upon the dip solder process in use.
Standard dip solder methodology in use by many manufacturers entails the use of an Alpha 611 flux manufactured by Alpha Metals Incorporated and the use of a eutectic solder combination that includes 62 parts tin, 36 parts of lead, and 2 parts of silver. Dip soldering then takes place at a temperature of between 245 and 260 degrees centigrade for a period of from 4 to 6 seconds. This methodology, however, typically may produce one of the two above stated undesirable conditions. It has been found that in order to achieve acceptable wetting characteristics one or more of the following actions were required. The first that it be necessary to reduce the solder temperature; second, increasing the preheat time after flux is applied; and third, to reduce the flow of the solder wave; or four, decreasing the duration of the dip.
Each of the above solutions has proved to have a definite disadvantage in that they increase process control requirements, increase set up time, and result in high rates of circuit repair. This is true particularly since the solerability of the DuPont 6001 copper is marginal at best utilizing standard solder techniques. However, in accordance with the teachings of the present invention a technique has been developed which produces a substantially improved solder joints with industry requirements for the visual characteristics of solder fillets without the added processing sensitivity and yield fallout.